Cardiovascular disease is a leading cause of death, and as a result, the medical community has devised various methods and devices for the treatment of coronary heart disease. One such treatment utilized in cases involving atherosclerosis and/or other forms of coronary narrowing is referred to a percutaneous transluminal coronary angioplasty, sometimes simply referred to as angioplasty or PTCA. The objective of this technique is to radially enlarge the lumen of an impacted artery. This is accomplished by first positioning an expandable balloon in a target lesion (i.e., the narrowed lumen of the coronary artery) and then inflating it. Inflation of the balloon enlarges the lumen by causing (1) soft or fatty plaque deposits to be flattened and (2) hardened deposits to crack and split. In addition, the artery wall itself is stretched by the inflated balloon.
In a typical percutaneous transluminal coronary angioplasty (PTCA) procedure, a hollow guiding catheter is introduced into the cardiovascular system of a patient via a relatively large vessel such as the femoral artery in the groin area or the brachial artery in the arm. After access to the patient's cardiovascular system has been achieved, a short hollow sheath is inserted to provide a passageway during the procedure. After the guiding catheter is directed to the ostium of the coronary artery to be treated by angioplasty, a flexible guide wire and a dilatation catheter having a balloon on the distal end thereof are introduced into the guide catheter with the guide wire sliding through the dilatation catheter. The guide wire is advanced to a target lesion in the vasculature. A balloon or dilatation catheter (made of, for example, polyethylene, polyethylene terathalate, PEBAX (polyamide block copolymers and polyester block copolymers), polyvinyl chloride, polyolefin, nylon or other suitable substance) is then slidably advanced over the previously advanced guide wire until properly positioned across the target lesion. Radiopaque markers in the balloon portion of the dilatation catheter assist in the positioning of the balloon across the lesion. After proper positioning, the balloon is inflated (e.g. generally with a contrast material to permit fluoroscopic viewing during the treatment) to enlarge the lumen of the artery. Treatment may require that the balloon be alternately inflated and deflated until satisfactory enlargement has been achieved. Lastly, the balloon is deflated to a small profile so that the dilatation catheter may be withdrawn from the patient's vasculature, and blood flow may resume through the dilated artery. Unfortunately, after angioplasty procedures of this type, there may occur a restenosis of the artery; i.e. a renarrowing of the treated coronary artery that significantly diminishes any positive results of the angioplasty procedure. In the past, restenosis frequently necessitated additional PTCA procedures or even more drastic open-heart surgery.
To prevent restenosis and strengthen the target area, various devices have been proposed for mechanically keeping the affected vessel open after completion of the angioplasty procedure. Such mechanical endoprosthetic devices, generally referred to as stents, are typically inserted into the vessel, positioned across the target lesion, and then expanded and/or implanted to first increase and then maintain the diameter of the lumen. A stent is typically mounted in a compressed state around a deflated balloon, and the balloon/stent assembly maneuvered through a patient's vasculature to the site of a target lesion. Once in place, the balloon is inflated causing expansion of the stent to a larger diameter for placement and implantation in the vasculature. In general, such endoprosthetic devices effectively overcome restenosis, thereby permitting and maintaining an increased flow of blood through a vessel.
Ideally, a stent should be relatively flexible to facilitate delivery (e.g. through torturous body lumens) and radially stable such that when expanded, the lumen receives adequate support to maintain its appropriate diameter. Stents may include a plurality of axial bends or crowns adjoined together by a plurality of struts so as to form a plurality of U-shaped members which are, in turn, coupled together. Additionally, stents may be formed from an implantable biocompatible material (e.g. stainless steel, titanium, tantalum, super-elastic nickel-titanium alloys, high-strength thermoplastic polymers, etc.).
Stents may be manufactured by means of a number of different methods involving wire, ring regions, torroids, or tubes. One method of cutting tubing to produce a desired pattern is shown and described in U.S. Pat. No. 5,780,807 issued Jul. 14, 1998, and entitled “Method and Apparatus for Direct Laser-Cutting of Metal Stents”, the teaching of which are hereby incorporated by reference. This patent describes a method of producing a stent wherein a desired pattern is cut into the tubing by means of a machine controlled laser.
The tubing is fixed in a rotatable collet fixture of a machine-controlled laser apparatus so as to position the tubing relative to the laser beam. The tubing is then rotated and moved longitudinally relative to the laser in accordance with a predefined set of machine-encoded instructions. In this manner, the laser selectively removes material from the tubing by ablation, thus cutting a desired pattern into the tube and forming the stent.
Unfortunately, conventional laser-cutting methods such as the one mentioned above produce edges which are essentially perpendicular to the axis of the laser beam. Thus, cross-sections of stents produced in this manner are substantially square or rectangular. This is problematic because the resulting edges make insertion and manipulation of the stent more difficult and increase the likelihood of trauma to the patient's vasculature. Conversely, a stent comprised of fine geometries (e.g. narrow cuts and rounded edges) is easier to insert/manipulate minimizing risk to a patient. The edges produced by direct laser-cutting may be smoothed by electrochemically polishing the stent in an acidic aqueous bath (e.g. comprised of sulfuric acid, carboxylic acids, phosphates, corrosion inhibitors, and a biodegradable surface-active agent) at temperatures of approximately 110° Fahrenheit to 135° Fahrenheit. Unfortunately, such electropolishing processes present certain problems. For example, such techniques are generally cumbersome and messy. Furthermore, such processes are difficult to control resulting in lack of product consistency (e.g. stent-to-stent variations in stent element cross-section).
Segmented or modular stents have mitigated some of the problems associated with prior art stents; e.g. inability to conform to vessel shape, lack of sufficient flexibility for advancing through and implantation in vascular anatomy, etc. One such modular stent is shown and described in U.S. Pat. No. 5,817,152 issued Oct. 6, 1998 and entitled “Connected Stent Apparatus”, the teachings of which are hereby incorporated by reference. In this case, a single stent is comprised of at least two shorter stent segments which are connected, for example, by welding so as to produce a stent tailored to the length of the stenosis to be treated. Such stent segments may be produced from a machined wire ring or torroid (e.g. machined from stainless steel bar stock) which is then bent or formed into a desired shape, perhaps through the use of a forming tool. Unfortunately, since the diameter of the wire and the inner and outer diameters of the wire ring or torroid must be very precise and since the rings or torroids are typically produced one at a time, the machining process is difficult, time-consuming, and costly.
It should therefore be appreciated that it would be desirable to provide an improved method for the manufacture of a stent wherein the number of processing steps and overall production costs are reduced while maintaining the radial strength, flexibility, and edgeless geometry of the stent.